1. Field of the Invention
The present invention relates to a bipolar ion detector. More specifically, it relates to a bipolar ion detector capable of detecting both positive and negative energetic ions in a single configuration, without the need to change the electrical configuration, or use electro-optical isolation, to switch from detecting one polarity to detecting the other.
2. Description of the Related Art
A microchannel plate (MCP) detector consists of a two-dimensional array of very small diameter glass capillaries or channels, also called pores, which are fused together and sliced into a thin plate. A single incident particle, which may be an ion, electron, photon, etc., enters a channel. Upon impact with a channel wall, the collision dislodges an electron. The dislodged electrons are accelerated by an electric field developed by a voltage applied across both ends of the MCP. They travel until they strike the channel wall, thus producing more secondary electrons, with the cascade process yielding a cloud of several thousand electrons, which emerge from the rear of the plate. If two or more MCPs are operated in succession, a single input event will generate 100 million or more electrons at the output. The output signals are detected in a number of ways, including metal or multimetal anodes, resistive anode, wedge and strip anode, delay-line readout, and others.
Such standard MCP ion detectors are typically used to detect positive ions, due to the way in which they are electrically biased. For positive ions, the collection anode can be at ground potential which makes it very easy to electrically couple to high speed electronics. For negative ions, the entire analyzer has to be floated to high negative voltage or the anode must be floated to a high positive voltage (or a combination of both). Neither of these methods is easy to implement, thus stimulating a need to modify this design for the detection of both positive and negative ions.
In order to detect both positive and negative ions in a single detector, present technology utilizes electro-optical isolation. In this approach, ions enter the detector through a wire mesh grid where they are generally post-accelerated into the face of a MCP. The MCP serves to convert the ions into electrons. The primary electrons are multiplied as they move down the pores of the MCP where the gain is controlled by the voltage difference across the MCP. The electrons exit the MCP and are further accelerated by an electric field into a scintillator plate. The scintillator plate serves to convert the electrons into photons, where the gain of the conversion process is controlled by the voltage difference between the scintillator plate and the rear of the MCP. Then the photons are detected by a photomultiplier tube (PMT), with the gain on the PMT being controlled by the voltage applied to the PMT.
While these ion detectors accomplish their intended purpose, they suffer from a number of drawbacks. For the first standard MCP detector described above, it is necessary to change the electrical bias of the detector in order to switch between detection of positive and negative ions. The second technique of electro-optical isolation requires an intermediate step of using a scintillator plate to convert the ions into photons, then directs the photons into a PMT, which converts the photons back to electrons at ground potential. The hardware necessary to do this usually adds undesirable complexity, weight and length to the detector.
A related aspect of these designs is the use of post-acceleration to draw the ions into the front face of the MCP. The use of post acceleration makes it necessary to change the electrical configuration of the unit to accommodate the detection of positive and negative ions.
In order to overcome these problems, what is needed is a bipolar ion detector, which permits direct conversion of the ion signal into an amplified electron signal, without the need for changing the electrical bias or using electro-optical isolation, thus addressing and solving problems associated with conventional systems.